Conventionally, although crude oil and natural gas drilled from submarine oil fields and the like have been shunned because of severe working environments, the recent tight energy conditions bring about a situation in which the crude oil and natural gas must be utilized. Therefore, the demand for stainless steel which is excellent in pitting resistance, particularly duplex stainless steel, is increasing as a material for steel pipes or other structures used in seawater.
A so-called super duplex stainless steel which is enhanced in pitting resistance because it contains W in addition to the adjustment of the contents of Cr, Mo and N (nitrogen), which are generally effective for improving the pitting resistance of duplex stainless steel, is disclosed in Patent Document 1. It suggests that an index, showing the pitting resistance of duplex stainless steel, PREW of the following equation (B) containing W, in addition to PRE (pitting resistance equivalent) of the following equation (A).
The pitting resistance index PRE or PREW is adjusted to not less than 35 in the general duplex stainless steel and to not less than 40 in the super duplex stainless steel. Conventional techniques for improving the pitting resistance were performed based on how much the pitting resistance index PRE or PREW can be increased.PRE=Cr+3.3Mo+16N (nitrogen)  (A)PREW=Cr+3.3(Mo+0.5W)+16N (nitrogen)  (B)
In the equations (A) and (B), each chemical symbol shows the content of each element (% by mass).
The influence on the pitting resistance of non-metallic inclusions has not been examined in the duplex stainless steel. However, with respect to the pitting resistance of austenitic stainless steel, it is known that Mn sulfides are most harmful to the pitting resistance, and oxides thereof are harmless as described in Non-Patent Document 1.
Oxide-based inclusions contained in stainless steels are generally composite oxides composed of oxides such as Al oxide (Al2O3), Si oxide (SiO2), Cr oxide (Cr2O3). These oxides were assumed to have no influence on pitting because they hardly dissolve in aqueous solutions or so-called insolubility. On the other hand, although Ca and Mg, and further S which are impurity elements in steel product, might be contained in the oxides, the influence of these elements on the pitting resistance have been never examined.
[Patent Document 1] Japanese Patent Laid-Open No. H05-132741
[Non-Patent Document 1] J. E. Castle et al., “Studies by Auger Spectroscopy of Pit Initiation at the site of Inclusions in Stainless Steel”, Corrosion Science, Volume 30, No. 4/5, p. 409